Method and system for pre-purification of a feed gas stream

ABSTRACT

A system and method of pre-purification of a feed gas stream is provided that is particularly suitable for pre-purification of a feed air stream in cryogenic air separation unit. The disclosed pre-purification systems and methods are configured to remove substantially all of the hydrogen, carbon monoxide, water, and carbon dioxide impurities from a feed air stream and is particularly suitable for use in a high purity or ultra-high purity nitrogen plant. The pre-purification systems and methods preferably employ two or more separate layers of hopcalite catalyst with the successive layers of the hopcalite separated by a zeolite adsorbent layer that removes water and carbon dioxide produced in the hopcalite layers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application and claimspriority to U.S. patent application Ser. No. 17/264,445 filed on Jan.29, 2021 which claims the benefit of and priority to U.S. provisionalpatent application Ser. No. 63/067,539 filed on Aug. 19, 2020 thedisclosures of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a system and method for removingimpurities from a feed gas stream, and more particularly, to a methodand apparatus for removing water, carbon dioxide, hydrogen, and carbonmonoxide from a feed gas stream prior to its introduction into acryogenic distillation system. More specifically, the present inventionrelates to a system and method for pre-purification of a feed air streamin a cryogenic air separation unit.

BACKGROUND

Adsorption is well established technology for the purification of gasesand for the treatment of fluid waste streams. Purification andseparation of atmospheric air comprises one of the main areas in whichadsorption methods are widely used. For an increase of their efficiency,novel and improved pre-purification systems and methods are continuouslybeing developed.

One of the areas of strong commercial and technical interest representspre-purification of air before its cryogenic distillation. Conventionalair separation units for the production of nitrogen (N₂) and oxygen (O₂)and also for argon (Ar) by the cryogenic separation of air are basicallycomprised of two or at least three, respectively, integrateddistillation columns which operate at very low temperatures. Due tothese low temperatures, it is essential that water vapor (H₂O), andcarbon dioxide (CO₂) is removed from the compressed air feed to an airseparation unit to prevent freeze up of components within the airseparation unit.

Current commercial methods for the pre-purification of feed air includetemperature and/or pressure swing adsorption units that employ layers ofadsorbent materials together with optional catalytic pre-purificationtechniques. A pre-purification unit (PPU) situated upstream of thecryogenic distillation system is typically used that includes an upfrontadsorbent layer to remove water, carbon dioxide as well as hydrocarbonsand other contaminants including oxides of nitrogen. Such PPU may alsooptionally include one or more catalysts targeted to remove one or morecontaminants followed by a final adsorbent layer downstream of theoptional catalysts to remove the contaminants produced by the catalysisprocess.

If not removed, water and carbon dioxide present in the feed air willfreeze out and block heat exchangers employed for cooling the feed airprior to distillation in the cryogenic distillation columns. Removal ofhydrocarbons and nitrous oxides is often required to ensure the safeoperation of such cryogenic distillation systems that typically involveprocessing oxygen-rich streams.

Before entering the PPU, atmospheric air is typically compressed to anelevated pressure from about 0.45 MPa to 1.1 MPa, followed by acombination of cooling steps and removal of condensed water. The cooledfeed air stream is then passed to a PPU where any remaining water andcarbon dioxide are first removed by adsorption in a bed of a molecularsieve and/or activated alumina. The air stream exiting the bed ofmolecular sieve and/or activated alumina is substantially free of carbondioxide, water, hydrocarbons, and nitrous oxide. Preferably, to avoidfreeze-out, the content of water in the compressed and pre-purified airfeed stream must be less than 0.1 ppm (part per million) while thecontent of carbon dioxide in the compressed and pre-purified air feedstream must be less than 1.0 ppm. From a safety perspective, thecompressed and pre-purified air should be substantially free of heavyhydrocarbons and nitrous oxides.

In addition, some applications for the electronics industry and selectedother industries require the removal of hydrogen and/or carbon monoxidefrom the feed air stream before processing the feed air stream in thecryogenic distillation system to produce a high purity or ultra-highpurity nitrogen product. A conventional PPU having only a bed ofmolecular sieve and/or activated alumina is quite capable of removingcarbon dioxide, water, hydrocarbons, and nitrous oxide from the cooledfeed air. However, the activated alumina or molecular sieve are noteffective for the substantial removal of carbon monoxide or hydrogenthat may be present in the feed air.

Prior art techniques of removing carbon monoxide and hydrogen in suchapplications have used catalytic based pre-purification techniqueswithin the PPU. For example, pre-purification processes requiringremoval of hydrogen often use a noble metal containing catalyst such asa platinum or palladium containing catalyst material. Likewise, inapplications requiring removal of carbon monoxide or removal of bothcarbon monoxide and hydrogen use of catalytic materials such ashopcalite with or without noble metal containing catalysts. As usedherein, the term ‘hopcalite’ is not used as a tradename but rather isused generically to refer to a catalyst material that comprises amixture of copper oxide and manganese oxide.

For example, U.S. Pat. No. 6,048,509 discloses a method and processutilizing a modified precious metal catalyst (platinum or palladium) andat least one member selected from the group consisting of iron, cobalt,nickel, manganese, copper, chromium, tin, lead and cerium on alumina)for oxidation of carbon monoxide to carbon dioxide, followed by waterremoval in an adsorbent layer and carbon dioxide removal in a secondadsorbent layer. An option for further hydrogen removal is provided witha second noble metal containing catalyst layer followed by water removalin subsequent adsorbent layers.

Another example is highlighted in U.S. Pat. No. 5,906,675 (Jain) thatremoves carbon monoxide by means of a carbon monoxide oxidation catalystlayer (34a,34b,34c) such as single layer of hopcalite or other metaloxides and also removes hydrogen impurities using a hydrogen oxidationcatalyst layer (36a, 36b,36c) containing a noble metal based catalyst.This Jain reference goes on to teach that the carbon monoxide oxidationcatalyst layer can be disposed upstream or downstream of the hydrogenoxidation catalyst. Alternatively, Jain teaches that the carbon monoxideoxidation catalyst and the hydrogen oxidation catalyst can be combinedas a single mixed layer (see Column 5, lines 1-17). U.S. Pat. No.5,906,675 to Jain goes on to teach that a final carbon dioxide adsorbentlayer is disposed downstream of the carbon monoxide oxidation catalystand hydrogen oxidation catalyst layer(s) (See column 5, lines 17-33).

Another Jain reference, namely European Patent No. EP0904823 alsodiscloses a similar arrangement of a carbon dioxide adsorbing layerdisposed downstream of carbon monoxide oxidation catalyst and hydrogenoxidation catalyst layer(s) to clean up any remaining carbon dioxide. InEuropean Patent No. EP0904823, there is also a first adsorbent layer toadsorb water and carbon dioxide upstream of the carbon monoxideoxidation catalyst and hydrogen oxidation catalyst layer(s).

Yet another example is highlighted in U.S. Pat. No. 6,093,379 whichdiscloses a process for combined hydrogen and carbon monoxide removalconsisting of a first layer to adsorb water and carbon dioxide onalumina or zeolite, and a second layer of a precious metal catalyst(palladium on alumina) to simultaneously oxidize carbon monoxide, adsorbthe formed carbon dioxide and chemisorb hydrogen.

Other prior art references teach the use of other catalyst materialssuch as hopcalite to remove carbon monoxide and hydrogen. Two suchexamples of use of hopcalite for pre-purification to remove hydrogenfrom air are U.S. Patent Application Publication No. 2003/064014 (Kumaret al.) The Kumar et al. reference shows that it has been well known forover 20 years that a hopcalite catalyst removes hydrogen and carbonmonoxide from air and is particularly useful for removing both carbonmonoxide and hydrogen from a feed air stream during pre-purification incryogenic air separation units.

The closest prior art reference, however, is and U.S. Pat. No. 8,940,263(Golden, et al.) which discloses the use of a single layer of hopcalitefor removal of substantially all of the hydrogen and carbon monoxide.The examples in Golden et al. confirm what is taught in Kumar et al.that hydrogen is chemisorbed in the single layer of hopcalite materialsuch that the use of a longer bed of hopcalite catalyst which translatesto longer residence times of the dry gas in the hopcalite layergenerally improves the hydrogen chemisorption process in the hopcalitematerial.

While the above-identified prior art pre-purification systems andmethods target removal of impurities such as hydrogen, carbon monoxide,water, and carbon dioxide from feed air streams, the relative costsassociated with pre-purification systems and methods remain high,particularly systems and methods that employ the use of palladium basedcatalysts or other noble metal catalysts. Accordingly, there is acontinuing need to improve such pre-purification systems and processes,particularly to reduce the costs of such pre-purification withoutsacrificing performance by eliminating the use of palladium basedcatalysts or other noble metal catalysts. In other words, there is aneed for improved systems and methods for pre-purification of anincoming feed air stream to a cryogenic air separation unit, includingsubstantial removal of hydrogen, carbon monoxide, water and carbondioxide in the production of high purity or ultra-high purity nitrogenthat has cost advantages and performance advantages over prior artpre-purification systems and methods. In particular, there is a need toimprove the cost and performance of hopcalite based pre-purificationsystems such the Golden, et al. pre-purification system and method.

SUMMARY OF THE INVENTION

The present invention may broadly be characterized as a system (e.g.pre-purification unit) and method of purifying a feed stream to reducethe hydrogen and carbon monoxide impurities present in the feed streamthat represents clear improvements in costs and/or performance over theprior art pre-purification systems and methods. In particular, byeliminating the use of palladium and other noble metal based catalystsor other noble metal catalysts from such pre-purification systemsrepresents tremendous value. Also, using specific layering arrangementswithin the pre-purification unit and associated methods, wherein carbondioxide is removed using one or more intermediate layers disposedbetween layers of carbon monoxide oxidation catalysts and hydrogenoxidation catalysts, the hydrogen removal performance of thepre-purification unit is greatly enhanced.

More specifically, the present method of purifying a feed stream toreduce the hydrogen and carbon monoxide impurities comprising the stepsof: (a) passing the feed stream through at least one layer of adsorbentconfigured to remove water and carbon dioxide from the feed stream andyield a dry feed stream substantially free of water and carbon dioxide;(b) passing the dry feed stream through a first layer of manganese oxideand copper oxide containing catalyst configured to remove at least someof the carbon monoxide and hydrogen from the dry feed stream and producea first intermediate effluent stream; (c) passing the first intermediateeffluent stream through a first intermediate layer disposed downstreamof the first layer of manganese oxide and copper oxide containingcatalyst, the first intermediate layer configured to remove at leastcarbon dioxide from the first intermediate effluent stream and produce asecond intermediate effluent stream; and (d) passing the secondintermediate effluent stream through a second layer of manganese oxideand copper oxide containing catalyst disposed downstream of the firstintermediate layer and configured to remove at least hydrogen from thesecond intermediate effluent stream to yield third intermediate effluentstream. The third effluent stream is subsequently passed through one ormore further layers of adsorbent configured to remove water and carbondioxide and yield a purified stream. Preferably, the first intermediatelayer comprises a molecular sieve layer or a layer of alumina whereasthe at least one adsorbent layer comprises a molecular sieve layer or alayer of alumina or both a molecular sieve layer and a layer of alumina.

The present invention may alternatively be characterized as a system orpre-purification unit for purifying a feed stream to reduce the hydrogenand carbon monoxide impurities present in the feed stream, the system orpre-purification unit comprising: (i) at least one layer of adsorbentconfigured to remove water and carbon dioxide from the feed stream andyield a dry feed stream substantially free of water and carbon dioxide;(ii) a first layer of manganese oxide and copper oxide containingcatalyst configured to remove at least some of the carbon monoxide andhydrogen from the dry feed stream and produce a first intermediateeffluent stream; (iii) a first intermediate layer disposed downstream ofthe first layer of manganese oxide and copper oxide containing catalyst,the first intermediate layer configured to remove at least carbondioxide from the first intermediate effluent stream and produce a secondintermediate effluent stream; (iv) a second layer of manganese oxide andcopper oxide containing catalyst configured to remove at least hydrogenfrom the second intermediate effluent stream to yield a thirdintermediate effluent stream; and (v) one or more further layers ofadsorbent configured to remove water and carbon dioxide from the thirdintermediate effluent stream yield a purified stream substantially freeof at least water, carbon dioxide, carbon monoxide and hydrogen.

Advantageously, the resulting purified stream purified stream issubstantially free of water, carbon dioxide, carbon monoxide andhydrogen throughout the entire cycle time typically used withpre-purification units of air separation plants. More importantly, theabove-described pre-purification unit and associated methods do not useany noble metal catalysts, and specifically no palladium catalystsresulting in significant capital cost savings compared topre-purification units that employ such palladium catalysts or othernoble metal catalysts to remove hydrogen.

The above-described methods of purification and pre-purification unitmay also include additional process steps and/or one or more additionallayers of manganese oxide and copper oxide containing catalysts as wellas one or more additional adsorbent layers disposed between the catalystlayers to remove any water and carbon dioxide exiting the catalystlayers.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with one or more claims specificallypointing out the subject matter that Applicants regard as the invention,it is believed that the present systems and methods for pre-purificationof a feed gas stream will be better understood when taken in connectionwith the accompanying drawing in which:

FIG. 1 depicts a partial, cross-section view of a section of apre-purification unit or pre-purification vessel suitable for use inpre-purification of a feed air stream in a cryogenic air separationunit;

FIG. 2 depicts a partial, cross-section view of a section of analternate embodiment of a pre-purification unit or pre-purificationvessel also suitable for use in pre-purification of a feed air stream ina cryogenic air separation unit;

FIG. 3 depicts a partial, cross-section view of a section of yet anotherembodiment of a pre-purification unit or pre-purification vessel alsosuitable for use in pre-purification of a feed air stream in a cryogenicair separation unit;

FIG. 4 is a graph that shows experimental data relating to hydrogenremoval as a function of time for certain hopcalite catalysts;

FIG. 5 is a bar chart that shows experimental data relating to effect ofcarbon monoxide and carbon dioxide on hydrogen removal for certainpalladium based catalyst materials after an 8 hour cycle time;

FIG. 6 is a schematic illustration of the experimental test set-up usedto compare the performance of a single layer of hopcalite catalyst to asplit-layer arrangement having two layers of hopcalite catalystsseparated by an intermediate layer of a molecular sieve;

FIG. 7 is a graph that shows experimental data comparing the hydrogenbreak-through as a function of time for a single layer of hopcalitecatalyst and a split-layer arrangement having two layers of hopcalitecatalysts separated by an intermediate layer of a molecular sievecertain hopcalite catalysts where the total residence time in thehopcalite catalyst layers is about 3.0 seconds;

FIG. 8 is a graph that shows experimental data comparing the carbondioxide and carbon monoxide break-through as a function of time for asingle layer of hopcalite catalyst and a split-layer arrangement wherethe total residence time in the hopcalite catalyst layers is about 3.0seconds;

FIG. 9 is a graph that shows experimental data comparing the hydrogenbreak-through as a function of time for a single layer of hopcalitecatalyst and a split-layer arrangement where the total residence time inthe hopcalite catalyst layers is about 1.4 seconds; and

FIG. 10 is a graph that shows experimental data comparing the carbondioxide and carbon monoxide break-through as a function of time for asingle layer of hopcalite catalyst and a split-layer arrangement wherethe total residence time in the hopcalite catalyst layers is about 1.4seconds.

DETAILED DESCRIPTION

The present system and method for pre-purification embodies a processfor removing gaseous impurities from a feed gas stream and is targetedfor applications where the purified stream is subsequently introducedinto a cryogenic distillation column such as cryogenic air separation.The disclosed pre-purification process comprises an adsorption andcatalyst based process for removing water, hydrogen, carbon monoxide andcarbon dioxide as well as other impurities from the feed stream gas.

The process comprises passing a feed stream gas containing theseimpurities through a multi-layer pre-purification vessel that ischaracterized as comprising at least three purification sectionsarranged in an adjacent manner such that the gas stream to be purifiedflows sequentially from the first purification section to the secondpurification section, and then to the third purification section alldisposed within the pre-purification vessel. It is understood that thearrangement of the three purification sections and the individual layersof materials within each section may be oriented such that the flow isin an axial orientation of the pre-purification vessel or may beoriented such that the flow is in an radial direction within thepre-purification vessel. It is also understood that pre-purificationunits may include two or more pre-purification vessels in which at leastone of the pre-purification vessels is used for pre-purification serviceremoving impurities from the feed gas stream while at least one otherpre-purification vessel is being regenerated, preferably with a purge orregeneration gas stream. The beds switch between pre-purificationservice and regeneration service periodically.

The first purification section of the pre-purifier vessel is configuredto remove impurities such as water, carbon dioxide, and optionally otherimpurities such as heavy hydrocarbons and oxides of nitrogen. The firstpurification section of the pre-purifier vessel may be comprised of amolecular sieve or one or more layers of adsorbents such as activatedalumina, silica gel or an X type zeolite such as NaX zeolite. Theindividual layers layer may also be a composite of these materials. Forremoval of hydrocarbon impurities a hydrocarbon adsorbent is oftenselected from the group consisting of types A and X zeolites and silicagel. Likewise, where removal of oxides of nitrogen are required, anadsorbent layer may include A, X, or Y type zeolites.

The second purification section of the pre-purifier vessel is configuredto remove carbon monoxide and hydrogen from the gas stream exiting thefirst purification section, with the carbon monoxide preferably removedvia catalysis and adsorption while the hydrogen generally removed bychemisorption, adsorption and catalysis. The degree to which hydrogen isremoved by catalysis or via adsorption and/or chemisorption depends onthe materials used within individual layers of the second purificationsection.

The third purification section of the pre-purifier unit is configured tofurther remove any water and carbon dioxide that exit the secondpurification section to produce a pre-purified gas stream substantiallyfree of water, carbon dioxide, carbon monoxide, hydrogen, and otherimpurities. Similar to the first purification section, the thirdpurification section may be comprised of one or more layers ofadsorbents such as activated alumina, silica gel or an X type zeolitesuch as NaX zeolite. Individual layers layer may also be a composite ofsuch materials.

As used herein, the phrase substantially free of hydrogen is a relativeterm that depends on the hydrogen content in the feed gas. For airpre-purification in a cryogenic air separation unit, substantially freeof hydrogen would typically mean less than about 500 ppb hydrogen orless than 20% of the hydrogen content in the feed gas, whicheverconcentration is lower. Likewise, the phrase substantially free ofcarbon monoxide is also a relative term that depends on the carbonmonoxide content in the feed gas and for air pre-purificationapplications typically would mean less than about 50 ppb carbon monoxideor less than 10% of the carbon monoxide content in the feed air,whichever concentration is lower. Substantially free of carbon dioxideand substantially free of water in air pre-purification applications forcryogenic air separation units are generally understood to mean aconcentration of 10 ppm or less.

The pre-purification vessel is configured to operate at the usual gasflows applicable for air separation units and well-known pressuresemployed for pre-purification of air in air separation units, generallyin the range of between about 0.2 bar(a) and about 25.0 bar(a) duringregeneration and/or purification steps. Likewise, the present system andmethod are designed to operate at temperatures that range from 5° C. to55° C. for the purification steps and temperatures as high as 200° C.for any regeneration steps. Tuning now to FIG. 1, there is shown apre-purification unit 10 comprised of a vessel 15 configured to receivea feed gas stream at inlet 20 and deliver a purified gas stream atoutlet 60. Within the vessel 15 there are shown seven (7) layers ofmaterials used to purify the feed gas stream. These seven (7) layers arebroadly characterized herein as defining three purification sections, asdescribed below.

The first purification section 30 of the pre-purifier unit 10 isconfigured to remove impurities such as water, carbon dioxide, andoptionally other impurities such as heavy hydrocarbons and oxides ofnitrogen. The first purification section 30 of the pre-purifier unit 10includes three layers of adsorbents such as activated alumina, silicagel or an X type zeolite such as NaX zeolite or combinations thereof,including adsorbent layers 32, 34, and 36.

The second purification section 40 includes a first catalyst layer ofhopcalite 41 configured to remove at least some of the carbon monoxideand some of the hydrogen from the dry feed stream exiting adsorbentlayer 36 and entering the second purification section 40. The secondpurification section 40 of the pre-purifier unit 10 further comprises asecond layer 43 disposed downstream of the first layer 41 configured toremove water and carbon dioxide from the gas stream exiting the firstlayer 41. This second layer 43 is preferably a zeolite layer. A thirdlayer depicted as another hopcalite catalyst layer 45 is disposeddownstream of the second layer 43 and is configured to further removehydrogen and carbon monoxide from gas stream exiting second layer 43.

The third purification section 50 of the pre-purifier unit 10 isconfigured to further remove any water and carbon dioxide that exit thesecond purification section 40 to produce a pre-purified gas streamsubstantially free of water, carbon dioxide, carbon monoxide hydrogen,and other impurities. The purified gas stream exits the pre-purifierunit 10 via outlet 60. The third purification section 50 is shown as onelayer 52 of adsorbent such as activated alumina, silica gel or an X typezeolite, or mixtures thereof.

A plurality of flat separation screens 70 are preferably installed flushto the vessel wall between the various hopcalite catalyst layers 41, 45and the adjacent adsorbent layers 36, 43, 52. The separation screens arepreferably made of Monel due to presence of high oxygen content in theregeneration gas.

In the alternate embodiment shown in FIG. 2, there is a pre-purificationunit 100 comprised of a vessel 115 configured to receive a feed gasstream at inlet 120 and deliver a purified gas stream at outlet 160.Within the vessel 115 there are shown nine (9) layers of materials usedto purify the feed gas stream, divided generally into three purificationsections. The first purification section 130 of the pre-purifier unit100 is configured to remove impurities such as water, carbon dioxide,and optionally other impurities such as hydrocarbons and oxides ofnitrogen in multiple layers 132, 134, and 136 of adsorbent materials,such as activated alumina, silica gel or an X type zeolite such as NaXzeolite or combinations thereof.

The second purification section 140 includes a first hopcalite catalystlayer 141 configured to remove at least some of the carbon monoxide andsome of the hydrogen from the dry feed stream exiting adsorbent layer136 and entering the second purification section 140. The secondpurification section 140 of the pre-purifier unit 100 further comprisesan adsorbent layer 143 disposed downstream of the first hopcalite layer141 configured to remove water and carbon dioxide from the gas streamexiting the first hopcalite layer 141. This adsorbent layer 143 ispreferably a zeolite layer. Another hopcalite catalyst layer 145 isdisposed downstream of the adsorbent layer 143 and is configured tofurther remove hydrogen and carbon monoxide from gas stream exiting thesecond layer 143. Another adsorbent layer 147 configured to remove waterand carbon dioxide from the gas stream exiting the second hopcalitelayer 145 is disposed downstream of the second hopcalite layer 145.Finally, a third hopcalite layer 149 configured to remove substantiallyall of the remaining hydrogen is disposed downstream of the adsorbentlayer 147. Similar to the embodiment of FIG. 1, a plurality of Monelseparation screens 170 are preferably installed between the varioushopcalite catalyst layers 141, 145, 149 and the adjacent adsorbentlayers 136, 143, 147, 152.

The third purification section 150 of the pre-purifier unit 100 is shownas one layer 152 of adsorbent such as activated alumina, silica gel oran X type zeolite, or mixtures thereof and is configured to furtherremove any water and carbon dioxide that exit the second purificationsection 140 to produce a pre-purified gas stream substantially free ofwater, carbon dioxide, carbon monoxide hydrogen, and other impurities.

In both embodiments depicted in FIGS. 1 and 2, the second purificationsections 40, 140 of the pre-purifier units 10, 100 may be broadlycharacterized as having two or more separate layers of hopcalite withthe successive layers of the hopcalite separated by a zeolite adsorbentlayer that removes water and carbon dioxide produced in the hopcalitelayers.

Tuning now to FIG. 3, there is shown yet another embodiment of thepresent system and method for pre-purification of a feed gas stream thatincludes a vessel 215 configured to receive a feed gas stream at inlet220 and deliver a purified gas stream at outlet 260. In the embodimentshown in FIG. 3, the first purification section 230 includes a layer ofalumina 233 and a layer of zeolite based molecular sieve 235 while thethird purification section 250 of the pre-purifier unit 200 isconfigured with a capping layer 252 of zeolite based molecular sieve toremove impurities such as water, carbon dioxide exiting the secondpurification section 240. As with the embodiments shown and describedwith reference to FIGS. 1 and 2, the various layers of adsorbentmaterials in the first and third purification sections may be activatedalumina, silica gel or an X type zeolites or combinations thereof toremove impurities such as water, carbon dioxide, and optionally otherimpurities in the gas streams flowing through such layers.

The second purification section 240 of the pre-purifier unit 200, on theother hand includes a hopcalite catalyst layer 241 configured to removemost of the carbon monoxide and some of the hydrogen from the dry gasstream exiting adsorbent layer 236 and entering the second purificationsection 240 followed by an adsorbent layer 243 disposed downstream ofthe hopcalite layer 241 configured to remove water and carbon dioxidefrom the gas stream exiting the hopcalite layer 241. This adsorbentlayer 243 is preferably a zeolite based molecular sieve. Different fromthe embodiments shown and described with reference to FIGS. 1 and 2, theembodiment shown in FIG. 3 includes a layer of noble metal catalyst suchas 0.5 wt % Pd/Al₂O₃ instead of additional hopcalite catalyst layers. Inthe layer of noble metal catalyst 244, and hydrogen from theintermediate gas stream exiting adsorbent layer 243 that has beencleansed of carbon dioxide and water is partially oxidized to waterand/or adsorbed by this catalyst layer 244 while any residual carbonmonoxide escaping the upstream hopcalite layer may also be oxidized inthis layer. Much of the water produced in the partial oxidation ofhydrogen may be adsorbed in the Al₂O₃ catalyst support while some of thewater and carbon dioxide produced will continue to the thirdpurification section 250. A plurality of Monel separation screens 270are preferably installed between the various hopcalite catalyst layers241 or catalyst layer 244 and the adjacent adsorbent layers 236, 243,252.

As is well known in the art, air pre-purification systems use two ormore pre-purification units or vessels so as to allow continuousproduction of purified air. When one or more of the pre-purificationunits is purifying the feed air, one or more other pre-purificationunits are being regenerated, preferably using a process widely known asthermal regeneration. The thermal regeneration process acts to desorbsthe water and carbon dioxide from various layers in the pre-purifierunits while also restoring the hydrogen adsorption capacity of thehopcalite catalyst layers and other catalyst layers.

Thermal regeneration is preferably done using a multi-step process thatoften involves the following four steps: (i) depressurizing the vesselto lower pressures suitable for regeneration; (ii) heating the layerswithin the vessel to desorb the water and carbon dioxide from variouslayers and restore the hydrogen adsorption capacity of the hopcalitecatalyst layers and/or other catalyst layers; (iii) cooling the layerswithin the vessel back to temperatures suitable for the purificationprocess; and (iv) repressurizing the vessel back to the higher operatingpressures required for the purification process. While thermalregeneration is preferred, it is contemplated that the present systemand methods could be used with pressure swing adsorption basedpre-purifiers or even hybrid type pre-purifiers.

Thermal regeneration is preferably conducted at lower pressures such as1.0 to 1.5 bar(a) compared to the purification process and must beconducted at temperatures of at least 180° C., and more preferably attemperatures of about 190° C., or more, subject to appropriate safetyrequirements. The heating step in the thermal regeneration process istypically conducted by heating a purge gas to produce a stream of hotpurge gas which is fed to vessel via outlet 60, 160, 260 and whichtraverses the layers of the pre-purifier unit 10, 100, 200 in reverseorder compared to the above-described purification process. In manyapplications, the purge gas may be taken as a portion of the product gasor from waste gas from the distillation columns of the cryogenic airseparation unit. As the hot purge gas passes through the varioussections and layers of the pre-purifier unit 10, 100, 200, the catalystlayers and adsorbent layers are regenerated. The effluent purge gasexiting the pre-purification unit 10, 100, 200 via the inlet 20, 120,220 is typically vented. After the catalyst layers and adsorbent layersare heated and regenerated, the pre-purification unit is then cooledusing a cool purge gas generally at a temperature from about 10° C. upto 50° C. that flows through the pre-purification unit in the samedirection as the hot purge gas. After cooling, the vessel isrepressurized to the higher operating pressures required by thepurification process.

The regeneration steps are conducted as described for a predeterminedperiod of time, typically referred to as the cycle time after which theservice or functions of the pre-purification units are switched so thatvessels previously regenerating come “on line’ and initiates thepurification process while vessels previously purifying the feed air go“off-line’ and initiate the regeneration process. Typicalpre-purification cycle times for high-purity or ultra-high puritynitrogen producing air separation plants is between about 240 minutesand 480 minutes. In this manner, each pre-purification unit alternatesbetween purification service and regeneration service to maintaincontinuous production of purified air substantially free of carbondioxide, water, carbon monoxide, hydrogen and other impurities.

The pre-purifier vessels depicted in FIGS. 1-3. are preferably denseloaded. Dense loading provides the most consistent and uniform packingof adsorbents and catalysts with minimal leveling of the layersrequired. Furthermore, dense packing minimizes adsorbent settling. Suchdense packing for pre-purifiers designed for carbon monoxide andhydrogen removal is optional and may be utilized for all layers in thebed to ensure integrity and uniform depth. Because of relatively thinlayers in second purification section for removal of carbon monoxide andhydrogen, including multiple layers of hopcalite and adsorbent layers,as well as any noble metal based catalyst that may be used, it isimportant to minimize the shifting and/or settling of the layers inorder to maintain a uniform depth of the layers over the life of thepre-purifier unit.

Example 1

The embodiment of FIG. 1 has been evaluated using computer basedsimulations and models to demonstrate the expected performance of thepre-purification unit and Table 1 shows modeling data for the gasstreams entering the pre-purifier and exiting each individual layer ofmaterial in the pre-purifier unit. The flow rate through thepre-purifier is modeled at about 99000 Nm³/h for a cycle time of between2 to 6 hours. For sake of brevity and simplicity, the followingdiscussion focuses on the second purification section of thepre-purifier unit in an effort to demonstrate performance and costbenefits of this arrangement compared to prior art pre-purificationsystems.

TABLE 1 Impurities (at Exit of Layer) CO H₂ H₂O CO₂ PPU Layer MaterialLength (ppb) (ppb) (ppm) (ppm) PPU Inlet — — 1000 1000 2000 450 Section1—Layer 1 Alumina  10 cm 1000 1000 2000 450 Section 1—Layer 2 Alumina 40 cm 1000 1000 10 450 Section 1—Layer 3 Zeolite 100 cm 1000 1000 <0.01<0.1 Section 2—Layer 4 hopcalite  18 cm <1 100 <1.0 1.0 Section 2—Layer5 Zeolite  20 cm <1 100 <0.01 <0.01 Section 2—Layer 6 hopcalite  10 cm<0.1 5 <0.1 <0.1 Section 3—Layer 7 Zeolite  20 cm <0.1 5 <0.01 <0.01 PPUOutlet — — <0.1 5 <0.01 <0.01

With reference to the data in Table 1 for this two-layer hopcalite basedarrangement, the initial hopcalite layer or first catalyst layer isabout 18 cm in length and configured to remove most of the carbonmonoxide via an oxidation with the copper and magnesium oxides in thecatalyst layer to produce carbon dioxide most of which may be adsorbedin the hopcalite layer and some of which exits the first catalyst layer.Concurrently, a first portion of the hydrogen in the gas streamtraversing the first catalyst layer is oxidized to produce water while asecond portion of the hydrogen in the gas stream traversing the firstcatalyst layer is adsorbed in the first catalyst layer while a thirdportion of the hydrogen impurities in the gas stream traversing thefirst catalyst layer passes through the first catalyst layer. Thehydrogen and carbon monoxide profiles depicted in Table 1 shows adistinct increase in water impurities and carbon dioxide in the gasstream exiting the first catalyst layer (i.e. not absorbed in the firstcatalyst layer), presumably from oxidation of the hydrogen and carbonmonoxide, respectively. The hydrogen reduction from about 1000 ppb to100 ppb is a net reduction of about 90% while the carbon monoxide showsa net reduction of about 99.9% from about 1000 ppb to about 1.0 ppb.Note that the hydrogen impurities are being removed in this firsthopcalite layer at an average rate of 5.0% per cm of hopcalite.

The second layer in the second section of the pre-purifier unit is azeolite based adsorbent about 20 cm in length that removes the waterimpurities from about 1.0 ppm to about 0.01 ppm and also removes carbondioxide from about 1.0 ppm to less than about 0.01 ppm.

The third layer in the second section of the pre-purifier unit is asecond hopcalite layer about 10 cm in length that receives the gasstream cleansed of water and carbon dioxide exiting the second layer andis configured to further remove the hydrogen from about 100 ppm to about5 ppm for a reduction of about 95% of the remaining hydrogen and removesany remaining carbon monoxide to levels below 0.1 ppb resulting in a gassubstantially free of carbon monoxide. Put another way, in this thirdlayer the hydrogen impurities are being removed at an average rate of9.5% per cm of hopcalite compared to an average rate of hydrogen removalof 5.0% per cm of hopcalite in the first layer of hopcalite catalyst.Again, the removal of hydrogen and substantially all of the carbonmonoxide yields an exit gas with less than about 0.1 ppm water and lessthan about 0.1 ppm of carbon dioxide, with much of the produced waterand carbon monoxide being adsorbed in the third layer (i.e. secondhopcalite layer).

Example 2

The embodiment of FIG. 2 has also been evaluated using computer basedsimulations and models to also demonstrate the expected performance ofthe depicted multi-layer hopcalite based pre-purification unit and Table2 shows modeling data for the gas streams entering the pre-purifiershown and described with reference to FIG. 2. As with the discussionabove regarding the data in Table 1, the materials are the same as inthe previous example and the flow rate of the air through thepre-purifier is also modeled at 99000 Nm³/h for a cycle time of between2 hours and 6 hours.

TABLE 2 Impurities (at Exit of Layer) CO H₂ H₂O CO₂ PPU Layer MaterialLength (ppb) (ppb) (ppm) (ppm) PPU Inlet — — 1000 1000 2000 450 Section1—Layer 1 Alumina  10 cm 1000 1000 2000 450 Section 1—Layer 2 Alumina 40 cm 1000 1000 10 450 Section 1—Layer 3 Zeolite 100 cm 1000 1000 <0.01<0.1 Section 2—Layer 4 hopcalite  10 cm <1 321 <0.7 1.0 Section 2—Layer5 Zeolite  20 cm <1 321 <0.01 <0.01 Section 2—Layer 6 hopcalite  10 cm<0.1 60 <0.3 <0.1 Section 2—Layer 7 Zeolite  10 cm <0.1 60 <0.01 <0.01Section 2—Layer 8 hopcalite  8 cm <0.1 5 <0.05 <0.01 Section 3—Layer 9Zeolite  10 cm <0.1 5 <0.01 <0.01 PPU Outlet — — <0.1 5 <0.01 <0.01

With reference to the data in Table 2 and focusing on the second sectionof the pre-purifier unit which comprises a multi-layer hopcalite basedarrangement, the initial hopcalite layer or first catalyst layer in thesecond purification section, identified as layer 4 in Table 2, is about10 cm in length and configured to remove most of the carbon monoxide viaan oxidation in the catalyst layer to produce carbon dioxide most ofwhich may be adsorbed in hopcalite layer and some of which exits thefirst catalyst layer. Concurrently, a first portion of the hydrogen inthe gas stream traversing the first catalyst layer is oxidized toproduce water while a second portion of the hydrogen in the gas streamtraversing the first catalyst layer is adsorbed while a third portion ofthe hydrogen impurities in the gas stream traversing the first catalystlayer passes through the first catalyst layer. The hydrogen and carbonmonoxide profiles depicted in Table 2 shows a distinct increase in waterimpurities and carbon dioxide in the gas stream exiting the firstcatalyst layer (i.e. impurities not absorbed in the first catalystlayer), presumably produced from oxidation of hydrogen and carbonmonoxide, respectively. The hydrogen reduction from about 1000 ppb to321 ppb is a net reduction of only about 68% while the carbon monoxideshows a net reduction of about 99.9% from 1000 ppb to about 1.0 ppb.

The second layer in the second section of the pre-purifier unitidentified as layer 5 is a zeolite based adsorbent about 20 cm in lengththat removes the water impurities from about 0.7 ppm to about 0.01 ppmand removes carbon dioxide from about 1.0 ppm to less than 0.01 ppm.

The third layer in the second section of the pre-purifier unitidentified as layer 6 is another hopcalite layer about 10 cm in lengththat receives the gas stream cleansed of water and carbon dioxideexiting the second layer and is configured to further remove thehydrogen from about 321 ppm to about 60 ppm for a reduction of about 81%of the remaining hydrogen and removes any remaining carbon monoxide tolevels below 0.1 ppb resulting in a gas substantially free of carbonmonoxide. Again, the removal of hydrogen and substantially all of thecarbon monoxide yields an exit gas with about 0.3 ppm water and up toabout 0.1 ppm of carbon dioxide, with much of the produced water andcarbon monoxide being adsorbed in the third layer (i.e. second hopcalitelayer).

The fourth layer in the second section of the pre-purifier unitidentified as layer 7 is another zeolite based adsorbent that againremoves the water impurities from about 0.3 ppm back down to about 0.01ppm level and removes carbon dioxide from about 0.1 ppm to less than0.01 ppm levels. This fourth layer of zeolite based adsorbent is onlyabout 10 cm in length, thus helping reduce cost of materials.

The fifth layer in the second section of the pre-purifier unit isidentified as layer 7 and is yet another hopcalite layer of only about8.0 cm in length that receives the gas stream exiting the fourth layerand is configured to further remove most of the remaining hydrogenimpurities from about 60 ppb level to about 5 ppb level for a reductionof about 92% of the remaining hydrogen to yield an effluent gas that issubstantially free of both hydrogen and carbon monoxide.

Advantageously, by using this multi-layer arrangement with multiplelayers of hopcalite separated by intermediate layers of an adsorbentconfigured to remove water and carbon dioxide, there is a noticeableimprovement in hydrogen removal capacity. In the modeled arrangement,the first hopcalite layer is 10 cm in length and removes 68% of thehydrogen in the stream traversing that first hopcalite layer whereas thesecond hopcalite layer is also 10 cm in length yet removes 81% of thehydrogen in the stream traversing that second hopcalite layer. The thirdhopcalite layer is only about 8 cm in length yet removes about 92% ofthe hydrogen in the stream traversing that third hopcalite layer. Inthis manner, hydrogen removal is performed in a cascading manner wherethe efficiency of hydrogen removal improves in successive hopcalitelayers.

Without being bound by any particular theory or design limitations,using a multi-layer, hopcalite based pre-purifier design with thiscascading hydrogen removal, one can improve hydrogen removal bydesigning the first hopcalite layer to remove between 50% and less than90% of the hydrogen in the feed stream, and in some embodiments removebetween 50% and less than 75% of the hydrogen in the feed stream. Thelast hopcalite layer is preferably configured to remove more than 90% ofthe hydrogen entering the last hopcalite layer. Intermediate hopcalitelayers, if used, are preferably designed or configured to removerelatively more hydrogen than the preceding hopcalite layer, measured asa percentage of hydrogen in the gas stream entering the hopcalite layer.Intermediate hopcalite layers can be preferably configured to removebetween 51% and 89% of the hydrogen entering that intermediate hopcalitelayer.

Example 3

FIG. 4 shows a graph of data obtained from a plurality of laboratorytests showing the hydrogen removal characteristics of a hopcalitecatalyst, specifically a 22.86 cm long bed of Carulite®. The laboratorytests passed air at a temperature of 20° C. (i.e. curve 301) or 40° C.(i.e. curves 302 and 303), a pressure of about 9.6 bar(a), and a flowrate of 13.3 slpm. The feed air stream had 3 ppm of hydrogen and either10 ppm of carbon monoxide (i.e. curves 301 and 302) or 1 ppm of carbonmonoxide (i.e. curve 303).

As seen in FIG. 4, the ratio of hydrogen concentration exiting the bedof Carulite catalyst to the hydrogen concentration entering the bed ofCarulite catalyst as a function of time is shown for three multipledifferent conditions. Curve 301 represents test conditions with the airstream at 20° C., the hydrogen level at the inlet of 3 ppm and thecarbon monoxide level at the inlet of 10 ppm and shows a hydrogen ratioof about 0.7 after 100 minutes and about 0.85 after 350 minutes.Increasing the temperature to 40° C., while keeping the 3 ppm hydrogenlevel at the inlet and 10 ppm carbon monoxide level at the inletimproves the hydrogen removal performance as shown in curve 302.Specifically, curve 302 shows a hydrogen ratio of about 0.4 after 100minutes and about 0.7 after 350 minutes. Curve 303 shows even betterhydrogen removal performance with the feed air temperature at 40° C. butreducing the carbon monoxide concentration in the feed air to 1 ppmwhile keeping the 3 ppm hydrogen impurity level at the inlet.Specifically, curve 303 shows a hydrogen ratio of about 0.3 after 100minutes and about 0.5 after 350 minutes. The improved hydrogen reductionbetween curves 302 and 303 suggests that the hydrogen removalperformance in a second and/or third layers of hopcalite 45, 145, 149 inthe embodiments shown in FIGS. 1 and/or 2 will be improved because thecarbon monoxide impurity levels entering the second and/or third layersof hopcalite is less than 1 ppm as shown in Tables 1 and 2.

Example 4

FIG. 5 shows a graph of data obtained from a plurality of laboratorytests showing the hydrogen removal characteristics of a 22.86 cm longbed of a palladium based catalyst such as 0.5 wt % Pd/Al₂O₃. In thisexample, synthetic air at a pressure of about 11 bar(a), a temperatureof 10° C. is passed through a tube containing the palladium basedcatalyst for a cycle time of about 480 minutes. The feed air stream had3 ppm of hydrogen impurities and either 1 ppm of carbon monoxideimpurities (i.e. bar 401) or 1 ppm of carbon dioxide (i.e. bar 402) orno carbon monoxide and no carbon dioxide impurities (i.e. curve 403).The height of each bar is representative of the average hydrogenbreakthrough at the end of the 480 minute cycle across multiple tests.

As seen in FIG. 5, the average hydrogen concentration exiting the tubeof catalyst at the end of the 480 minute cycle when the feed air has 3ppm hydrogen and 1 ppm carbon monoxide is over 50 ppb (i.e. bar 401).Comparatively, the average hydrogen concentration exiting the tube ofcatalyst at the end of the 480 minute cycle when the feed air has 3 ppmhydrogen and 1 ppm carbon dioxide is just over 20 ppb, suggestinghydrogen removal in the catalyst is improved if the treated stream hasless carbon monoxide. However, the average hydrogen concentrationexiting the tube of catalyst at the end of the 480 minute cycle when thefeed air has 3 ppm hydrogen and little or no carbon dioxide or carbondioxide is less than 5 ppb, suggesting hydrogen removal in the catalystis improved if the treated stream has substantially no carbon dioxideand no carbon monoxide.

The improved hydrogen reduction shown in bar 403 compared to hydrogenreduction depicted by curves 401 and 402 further suggests that thehydrogen removal performance in a palladium based catalyst layer 244 inthe embodiment of FIG. 3 will be improved by the presence of adsorbentlayer 243 which removes carbon dioxide and water before the gas streamenters the palladium catalyst layer. In addition, this data alsosuggests that the hydrogen removal performance in the second and/orthird layers 45, 145, 149 of hopcalite in the embodiments shown in FIGS.1 and/or 2 will be improved because of the prior hopcalite layers andadjacent adsorbent layers causing the gas stream entering the secondand/or third layer of hopcalite layers to be substantially free ofcarbon monoxide and carbon dioxide as shown in Tables 1 and 2.

Example 5

Examples 1-3 above discuss computer simulations and laboratory teststhat clearly suggest and/or demonstrate that a multi-layer hopcalitearrangement with the intermediate layer improves the hydrogen removalcapability. In further support of those results, Applicants furtherconducted several direct comparison tests of a single layer of hopcalitearrangement to a two layer hopcalite arrangement separated by a layer ofmolecular sieve, which Applicants refer to it as a ‘split layer’arrangement. To demonstrate that two layers of hopcalite separated by alayer configured to remove carbon dioxide disposed between the twohopcalite layers is a clear improvement over a single hopcalite layer ofthe type disclosed in Golden, et al. having the same total amount ofhopcalite, a series of comparative tests were run.

As shown in FIG. 6, Test Bed #1 610 having a 6 inch bed of 4×8 hopcalitecatalyst 611 (Carulite from Carus Corporation) 611 and havingapproximately 27 grams of catalyst was compared to a split bedarrangement 620 having a first 3 inch bed of 4×8 hopcalite catalyst(Carulite from Carus Corporation) 622 followed by a 3 inch bed of 8×12APG-III Molecular Sieve 624 and a second 3 inch bed of 4×8 hopcalitecatalyst (Carulite from Carus Corporation) 626 with the split bedarrangement 620 also having a total of ˜27 grams of hopcalite catalyst(TestBed #2). Feed conditions to TestBed #1 and TestBed #2 comprisedsynthetic air streams 611, 621 at about 10 bara and 25° C. and with 3ppm of hydrogen and 10 ppm of carbon monoxide added as contaminants. Theflow rate of the synthetic air streams 611, 621 was varied to simulatetwo different residence times in the hopcalite catalyst beds. At a firstflow rate of 5.8 standard liters per minute (slpm), the residence timethrough the full 6 inches of Carulite was a total of 3.0 seconds or 1.5seconds in each of the 3 inch layers of Carulite in the split layerarrangement. At a second flow rate of 12.4 standard liters per minute(slpm), the residence time through the full 6 inches of Carulite was atotal of 1.4 seconds or 0.7 seconds in each of the 3 inch layers ofCarulite in the split layer arrangement. Regeneration of the test bedsin all test runs during this experiment was set at 200° C. for 3 hours.A summary of the test conditions are shown in Table 2 below and theresults, expressed in breakthrough concentrations of hydrogen, carbondioxide and carbon monoxide in the effluent streams 619, 629 as afunction of time are depicted in FIGS. 7-10.

TABLE 3 Feed Feed Feed Total Flow H2 CO Residence Rate Inlet Inlet Timethru Test # and (Ref #) (slpm) (ppm) (ppm) Bed (s) Group D - Baseline 6″Carulite Bed (Test Bed #1) D-02-01 (701, 801) 5.8 3.0 10.0 3.0 D-02-02(702) 5.8 3.0 10.0 3.0 D-03-01 (901, 1001) 12.4 3.0 10.0 1.4 D-03-02(902) 12.4 3.0 10.0 1.4 Group E - 3″ Carulite - 3″ Sieve - 3″ Carulite(Test Bed #2) E-01-01 (711, 811, 821) 5.8 3.0 10.0 4.5 E-01-02 (712) 5.83.0 10.0 4.5 E-02-01 (911, 1011, 1021) 12.4 3.0 10.0 2.1 E-02-02 (912)12.4 3.0 10.0 2.1

The results of the direct comparison tests of a single layer ofhopcalite arrangement to a or ‘split-layer’ arrangement, namely twolayers of hopcalite separated by a layer of molecular sieve were bothsurprising and unexpected as shown in FIGS. 7-10. The concentrations ofhydrogen, carbon monoxide and carbon dioxide in the effluent streams619, 629 exiting Test Bed #1 and Test Bed #2 were measured over a cycletime of about 6.5 hours.

Turning to FIG. 7, at the flow rate of 5.8 slpm which equates to a netresidence time of about 3.0 seconds through the hopcalite material ineach test bed, the hydrogen break-through concentrations for each testbed were very similar for the first two hours of the cycle. However, thehydrogen break-through and the rate of hydrogen breakthrough after about2 hours through the end of the 6.5 hour cycle was noticeably different.For example, at the 3 hour mark, the hydrogen break-through in thesingle 6 inch layer of hopcalite was almost 100 ppb (curves 701, 702)while the hydrogen break-through in the split layer arrangement was lessthan 50 ppb (curves 711, 712). At the 4 hour mark, the hydrogenbreak-through in the single 6 inch layer of hopcalite was almost 200 ppb(curves 701, 702) while the hydrogen break-through in the split layerarrangement was less than 70 ppb (curves 711, 712). Finally, at the 6.5hour mark, the hydrogen break-through in the single 6 inch layer ofhopcalite was over 500 ppb (curves 701, 702) while the hydrogenbreak-through in a split layer arrangement was about 200 ppb (curves711, 712).

Turning to FIG. 8, at the flow rate of 5.8 slpm which equates to a netresidence time of about 3.0 seconds through the hopcalite material ineach test bed, as expected, the carbon monoxide break-throughconcentrations for each test bed were very similar for the duration ofthe cycle (curve 821). In addition, the carbon dioxide break-throughconcentrations for the first 3.5 hours of the cycle for each test bedwere also very similar. Thereafter, the carbon dioxide break-through andthe rate of carbon dioxide breakthrough after about 3.5 hours throughthe end of the 6.5 hour cycle was noticeably different. For example, atthe 5 hour mark, the carbon dioxide break-through in the single 6 inchlayer of hopcalite was almost 1 ppm (curve 801) while the carbon dioxidebreak-through in the split layer arrangement was steady at about 0.3 ppm(curve 811). At the 6.5 hour mark, the carbon dioxide break-through inthe single 6 inch layer of hopcalite was over 2 ppm (see curve 801)while the carbon dioxide break-through in the split layer arrangementwas steady at about 0.3 ppm (see curve 811).

Turning now to FIGS. 9 and 10, at the flow rate of 12.4 slpm whichequates to a net residence time of about 1.4 seconds through thehopcalite material in each test bed, the hydrogen break-through for eachtest bed started deviating from one another in about 30 minutes whilethe carbon dioxide break-through concentrations for each test bedstarted deviating from one another in less than 1 hour. Again, thehydrogen break-through and the rate of hydrogen breakthrough in thesplit layer arrangement (curves 911, 912) was clearly superior to thehydrogen breakthrough in the single 6 inch layer of hopcalite (curves901, 902). Also, the carbon-dioxide break-through and the rate ofincrease in carbon dioxide breakthrough in the split layer arrangement(curve 1011) was remarkably superior to the carbon dioxide breakthroughin the single 6 inch layer of hopcalite (curve 801). The carbon monoxidebreak-through concentrations for each test bed were very similar for theduration of the cycle (see curve 1021).

Clearly, the ‘split bed’ arrangement is more efficient in hydrogenremoval than compared to a single hopcalite bed having the same amountof hopcalite as evidenced by the amount of hydrogen breakthroughrealized over the entire purification cycle. Quantifying the actualimprovement would be characterized by comparing the total area under thehydrogen curves over the entire 6.5 hour cycle time in FIGS. 7 and 9. Analternate comparison would be to assess the difference in hydrogenbreakthroughs at select points in the purification cycle time (thevertical difference in the curves). For example, at the longer residencetimes (i.e. 3.0 seconds in the hopcalite catalyst), there is not muchdifference in hydrogen removal after only 1 hour of cycle time but after4 hours of cycle time the ‘split bed’ arrangement shows 67% betterhydrogen capture performance while after a 6.5 hour cycle time, the‘split bed’ arrangement shows about 60% better hydrogen captureperformance, So one can broadly say hydrogen removal performance is upto 67% better at any given moment depending on cycle time, feed streamcontaminants, and other operating conditions and variables. Likewise, atshorter residence times (i.e. 1.4 seconds in the hopcalite catalyst),the ‘split bed’ arrangement shows up to 25% better hydrogen captureperformance at any given moment of time compared to than the singlelayer hopcalite arrangement, again, depending on feed streamcontaminants as well as other operating conditions and variables.

The vastly superior results provides clear technical benefits insituations where the residence time through the hopcalite catalyst islonger (e.g. 3.0 seconds) or shorter (e.g. 1.4 seconds). When using the‘split layer’ arrangement, the pre-purification unit (PPU) designer canreduce the amount of hopcalite material used to achieve the same levelof hydrogen removal at traditional PPU cycle times compared to a singlehopcalite layer arrangement, resulting in lower material costs. Thisoverly broad characterization of hydrogen capture performanceimprovement suggests the designer cannot reduce the amount of hopcalitematerials, perhaps between 20% and 50% to achieve similar hydrogenreduction performance—depending on residence times, cycle times, feedstream conditions, contaminant levels, and other operating conditions. A20% reduction in hopcalite catalyst could translate of cost savingsupwards of $150,000 or more in some air separation plants currentlyunder construction.

Alternatively, the PPU designer can use the same amount of hopcalitematerials and extend the cycle time of PPUs configured with the ‘splitlayer’ arrangement compared to PPUs configured with a single hopcalitelayer arrangement, to achieve the same level of hydrogen break-throughresulting in improved system reliability, operational costs andperformance. With further ‘split layer’ arrangement optimization, a PPUdesigner can likely achieve both a reduction in hopcalite material andan extension of cycle times compared to a single hopcalite layerarrangement. A third benefit of the ‘split layer’ arrangement is theelimination or reduction of the palladium catalyst layer which savessignificant costs, as the cost of many noble metal catalyst materials,including palladium catalyst materials continue to rapidly increase. Inmany air separation plants, achieving the required hydrogen removaltargets while eliminating the use of palladium based catalyst materialssaves significant capital costs, in many cases upwards of $1,000,000 ormore per air separation plant.

The demonstrated reduction in carbon dioxide break-through is yetanother superior technical benefit and allows the PPU designer to reducethe amount of adsorbent materials used downstream of the catalyst layersand in the final capping layer which offsets, or partially offsets, thematerial costs of the additional intermediate molecular sieve layer.

While the present methods have been described with reference to apreferred embodiment or embodiments, it is understood that numerousadditions, changes and omissions can be made without departing from thespirit and scope of the present invention as set forth in the appendedclaims.

What is claimed is:
 1. A method of purifying a feed stream to reduce thehydrogen and carbon monoxide impurities present in the feed stream, themethod comprising the steps of: (a) passing the feed stream through atleast one layer of adsorbent configured to remove water and carbondioxide from the feed stream and yield a dry feed stream substantiallyfree of water and carbon dioxide; (b) passing the dry feed streamthrough a first layer of manganese oxide and copper oxide containingcatalyst configured to remove at least some of the carbon monoxide andhydrogen from the dry feed stream and produce a first intermediateeffluent stream; (c) passing the first intermediate effluent streamthrough a first intermediate layer disposed downstream of the firstlayer of manganese oxide and copper oxide containing catalyst, the firstintermediate layer configured to remove at least carbon dioxide from thefirst intermediate effluent stream and produce a second intermediateeffluent stream; and (d) passing the second intermediate effluent streamthrough a second layer of manganese oxide and copper oxide containingcatalyst disposed downstream of the first intermediate layer andconfigured to remove at least hydrogen from the second intermediateeffluent stream to yield third intermediate effluent stream; wherein thefirst intermediate layer comprises a molecular sieve layer or a layer ofalumina.
 2. The method of claim 1, wherein the dry feed stream, thefirst intermediate effluent stream, the second intermediate effluentstream, and the third intermediate effluent stream are never in contactwith a noble metal catalyst.
 3. The method of claim 1, wherein the thirdintermediate effluent stream is substantially free of hydrogen andcarbon monoxide throughout a cycle time of 6.5 hours.
 4. The method ofclaim 3, further comprising the step of passing the third effluentstream through one or more further layers of adsorbent configured toremove water and carbon dioxide and yield a purified streamsubstantially free of at least water, carbon dioxide, carbon monoxideand hydrogen throughout the cycle time of 6.5 hours.
 5. The method ofclaim 4, wherein the dry feed stream, the first intermediate effluentstream, the second intermediate effluent stream, and the thirdintermediate effluent stream, and the purified stream are never incontact with a noble metal catalyst.
 6. The method of claim 4, whereinthe one or more further layers of adsorbent further comprise a molecularsieve layer or a layer of alumina or both a molecular sieve layer and alayer of alumina.
 7. The method of claim 1, wherein the at least oneadsorbent layer comprises a molecular sieve layer or a layer of aluminaor both a molecular sieve layer and a layer of alumina.
 8. The method ofclaim 1, wherein a residence time of the dry feed stream within thefirst layer of manganese oxide and copper oxide containing catalyst isless than or equal to 1.5 seconds.
 9. The method of claim 1, wherein aresidence time of the second intermediate effluent within the secondlayer of manganese oxide and copper oxide containing catalyst is lessthan or equal to 1.5 seconds.
 10. The method of claim 4, wherein thepurified stream comprises between 5 ppb hydrogen and 500 ppb hydrogen.11. The method of claim 4, wherein the purified stream comprises no morethan about 10 ppb hydrogen.
 12. The method of claim 4, wherein thepurified stream comprises no more than about 10 ppb carbon monoxide. 13.The method of claim 1, further comprising the steps of: passing thethird intermediate effluent stream through a second intermediate layerdisposed downstream of the second layer of manganese oxide and copperoxide containing catalyst, the second intermediate layer configured toremove at least carbon dioxide from the third intermediate effluentstream and produce a fourth intermediate effluent stream; and passingthe fourth intermediate effluent stream through a third layer ofmanganese oxide and copper oxide containing catalyst to produce a fifthintermediate effluent; wherein the second intermediate layer comprises amolecular sieve layer or a layer of alumina.
 14. The method of claim 13,wherein the fifth intermediate effluent stream is substantially free ofhydrogen and carbon monoxide throughout a cycle time of 6.5 hours. 15.The method of claim 14, further comprising the step of passing the fifthintermediate effluent stream through one or more further layers ofadsorbent configured to remove water and carbon dioxide and yield apurified stream substantially free of at least water, carbon dioxide,carbon monoxide and hydrogen throughout the cycle time of 6.5 hours. 16.The method of claim 15, wherein the one or more further layers ofadsorbent comprises a molecular sieve layer or a layer of alumina orboth a molecular sieve layer and a layer of alumina.
 17. The method ofclaim 13, further comprising the steps of: passing the fifthintermediate effluent stream through a third intermediate layer disposeddownstream of the third layer of manganese oxide and copper oxidecontaining catalyst, the third intermediate layer configured to removeat least carbon dioxide from the fifth intermediate effluent stream andproduce a sixth intermediate effluent stream; and passing the sixthintermediate effluent stream through a fourth layer of manganese oxideand copper oxide containing catalyst to produce a seventh intermediateeffluent wherein the third intermediate layer comprises a molecularsieve layer or a layer of alumina.
 18. The method of claim 17, whereinthe seventh intermediate effluent stream is substantially free ofhydrogen and carbon monoxide throughout a cycle time of 6.5 hours. 19.The method of claim 18, further comprising the step of passing theseventh intermediate effluent stream through one or more further layersof adsorbent configured to remove water and carbon dioxide from thestream substantially free of hydrogen and carbon monoxide and yield apurified stream substantially free of at least water, carbon dioxide,carbon monoxide and hydrogen throughout the cycle time of 6.5 hours. 20.The method of claim 19, wherein the one or more further layers ofadsorbent comprises a molecular sieve layer or a layer of alumina orboth a molecular sieve layer and a layer of alumina.
 21. The method ofclaim 1, wherein the feed stream is compressed air.
 22. The method ofclaim 1, wherein the dry feed stream is substantially free ofhydrocarbons and nitrous oxide.
 23. The method of claim 1, wherein thefeed stream comprises less than 20 ppm hydrogen.
 24. The method of claim1, wherein the feed stream comprises less than 50 ppm carbon monoxide.25. The method of claim 1, wherein the dry feed stream contains lessthan 10 ppm water.
 26. The method of claim 1, wherein the dry feedstream contains less than 10 ppm carbon dioxide.
 27. The method of claim1, wherein the second intermediate effluent contains no more than about10 ppm carbon dioxide.
 28. The method of claim 1, wherein the feedstream is at a pressure between 3 bar(a) and 30 bar(a).
 29. The methodof claim 1, wherein the feed stream is at a temperature between 0° C.and 70° C.
 30. A pre-purification unit for purifying a feed stream toreduce the hydrogen and carbon monoxide impurities present in the feedstream, the pre-purification unit comprising: at least one layer ofadsorbent configured to remove water and carbon dioxide from the feedstream and yield a dry feed stream substantially free of water andcarbon dioxide; a first layer of manganese oxide and copper oxidecontaining catalyst configured to remove at least some of the carbonmonoxide and hydrogen from the dry feed stream and produce a firstintermediate effluent stream; a first intermediate layer disposeddownstream of the first layer of manganese oxide and copper oxidecontaining catalyst, the first intermediate layer configured to removeat least carbon dioxide from the first intermediate effluent stream andproduce a second intermediate effluent stream; a second layer ofmanganese oxide and copper oxide containing catalyst configured toremove at least hydrogen from the second intermediate effluent stream toyield a third intermediate effluent stream; and one or more furtherlayers of adsorbent configured to remove water and carbon dioxide fromthe third intermediate effluent stream yield a purified streamsubstantially free of at least water, carbon dioxide, carbon monoxideand hydrogen.
 31. The pre-purification unit of claim 30, wherein thepre-purification unit contains no palladium catalyst material.
 32. Thepre-purification unit of claim 30, wherein the pre-purification unitcontains no noble metal containing catalysts.
 33. The pre-purificationunit of claim 30, wherein the first intermediate layer comprises amolecular sieve layer or a layer of alumina.
 34. The pre-purificationunit of claim 30, wherein the one or more further layers of adsorbentcomprise a molecular sieve layer or a layer of alumina or both amolecular sieve layer and a layer of alumina.